Creating lesions in and around the heart by radio frequency (RF) ablation is well known in the art of cardiac electrophysiology. It consists of applying RF energy, usually in the range of about 300-1200 kHz, to the inner surfaces of the heart by an electrode located at the distal section of a catheter. The RF energy generates heat in the heart tissue that is juxtaposed to the electrode. The heat kills the tissue and thereby destroys the electrochemical junctions between the cells and thus blocks the electrical conduction. The dead tissue is eventually replaced with scar tissue which is a very poor conductor of electrical current. Thus, a permanent blockage of a conduction pathway will occur.
RF ablation has worked very well for the treatment of supra ventricular tachycardias. These tachycardias are caused by discrete pathways which are easily pinpointed and destroyed by small focal lesions.
There are, however, other abnormal rhythms which include atrial flutter, atrial fibrillation, and other ventricular tachycardias whose electrical pathways range over broader areas. These abnormalities usually require a number of small lesions forming one or more continuous lines to completely ablate the conduction pathways causing the abnormality.
Unfortunately, a continuous lesion line is often difficult to generate with multiple small lesions. One reason for the difficulty is that the catheter must be guided from a remote entry site, such as the groin, while viewing the electrode on an X-ray monitor. It is very difficult to systematically make a number of small lesions in a continuous line from such a remote site. Another reason for the difficulty is that the heart is beating and constantly changing shape. Creating a continuous linear lesion by making a number of small focal lesions in a beating heart is almost impossible. Finally, difficulty lies in that the interior of the heart is irregularly shaped and covered with trabeculae.
In attempting to overcome these difficulties, some electrophysiologists have tried creating long linear lesions using a long cylindrical rigid electrode. However, the long cylindrical electrode has the problem of lack of flexibility. The problem arises because the distal section of the catheter has to be nearly straight to be introduced into the body and guided through the major blood vessels and into a heart chamber. Once in the heart chamber, the electrode almost always needs to be curved to achieve good contact with the heart tissue to be ablated. Thus, a rigid electrode is not practical.
Others have tried using a long thin strip shaped electrode that extends longitudinally in parallel to the axis of the catheter along the outside of the distal section. To ensure good adhesion to the catheter, the strip electrode needs to be glued or fastened to the catheter all along its entire length. Unfortunately, the glue or adhesive used impairs the flexibility of the distal section. Also, because the interior of the heart is irregularly shaped and covered with trabeculae, it is difficult to achieve good connection with heart tissue using the long thin strip shaped electrode. Many times contact with the catheter and the heart tissue occurs on the side of the catheter where the electrode is not physically located. A final problem with the strip electrode is that when the strip is attached to the catheter's distal section, the stiffness of the strip itself makes the catheter too rigid and the catheter does not bend properly.
Another attempt to create long linear lesions is to use closely spaced apart ring electrodes. If the ring electrodes lie on the catheter surface and are spaced apart just enough to give the tip some flexibility but not interrupt the continuity of the lesion, then the ring electrodes must be securely fastened to the catheter to prevent the electrodes from slipping off the catheter. The glue or adhesive used to fasten the electrodes will greatly diminish the flexibility of the end. On the other hand, if the ring electrodes are flush with the catheter surface and closely spaced apart in order to create a continuous linear lesion, the flexibility of the catheter will also be diminished do to the stiffness of the ring electrodes. Additionally, flush electrodes have problems getting good tissue contact to ablate the close spacing of the flush ring electrodes is such that there is not enough flexible catheter material between each electrode to afford any degree of flexibility. Another drawback with the multiple ring electrodes is that a separate lead wire would have to be attached to each one of the many individual ring electrodes. In catheters with small inner diameters, it is difficult to manufacture the catheters with many lead wires.
Another attempt at creating linear lesions is to use a coil of wire wrapped around the distal section of the catheter body. Unfortunately, this catheter also has many drawbacks. The wire is small in order to keep the catheter diameter small. This small diameter results in a high resistance compared to the interface of the surface of the coil and the tissue. In other words, it results in a circuit where the source resistance is high compared to the load resistance.
Accordingly, there is a need for a catheter that is useful in creating long linear lesions. One important requirement for the linear ablation electrode is that the electrode and distal section of the catheter remain flexible so that the electrode can be passed into a heart chamber and then be made to conform to the irregularly shaped heart surface. Another important requirement is that the electrode be fixed relative to the heart and not move during the ablation process so that providing RF energy to the heart will result in a continuous lesion without breaks.